diff --git a/doc/jsonrpc.qdoc b/doc/jsonrpc.qdoc
index 8f33b868..1cecb20e 100644
--- a/doc/jsonrpc.qdoc
+++ b/doc/jsonrpc.qdoc
@@ -31,6 +31,8 @@
 
     The JSONRPC API is self documenting and can be introspected by calling \c"JSONRPC.Introspect".
 
+    Parameters are optional if the type is the type is prefixed with "o:" for optional.
+
     \section1 Communicating with the server
     The server listens on TCP port 1234 for incoming TCP connections. It will respond to incoming
     connections with a some information about the server. Telnet can be used to issue commands for
diff --git a/libguh/devicemanager.cpp b/libguh/devicemanager.cpp
index bb7b7d21..09292fe0 100644
--- a/libguh/devicemanager.cpp
+++ b/libguh/devicemanager.cpp
@@ -121,10 +121,20 @@ QList<Vendor> DeviceManager::supportedVendors() const
     return m_supportedVendors.values();
 }
 
-/*! Returns all the supported \l{DeviceClass}{DeviceClasses} by all \l{DevicePlugin}{DevicePlugins} loaded in the system. */
+/*! Returns all the supported \l{DeviceClass}{DeviceClasses} by all \l{DevicePlugin}{DevicePlugins} loaded in the system.
+    Optionally filtered by vendorId. */
 QList<DeviceClass> DeviceManager::supportedDevices(const VendorId &vendorId) const
 {
-    return m_supportedDevices.values();
+    qDebug() << "returning devices" << vendorId;
+    QList<DeviceClass> ret;
+    if (vendorId.isNull()) {
+        ret = m_supportedDevices.values();
+    } else {
+        foreach (const DeviceClassId &deviceClassId, m_vendorDeviceMap.value(vendorId)) {
+            ret.append(m_supportedDevices.value(deviceClassId));
+        }
+    }
+    return ret;
 }
 
 /*! Add a new configured device for the given \l{DeviceClass} and the given parameters.
diff --git a/libguh/typeutils.h b/libguh/typeutils.h
index 8e89e788..0f30c628 100644
--- a/libguh/typeutils.h
+++ b/libguh/typeutils.h
@@ -1,6 +1,7 @@
 #ifndef TYPEUTILS_H
 #define TYPEUTILS_H
 
+#include <QMetaType>
 #include <QUuid>
 
 #define DECLARE_TYPE_ID(type) class type##Id: public QUuid \
@@ -10,12 +11,13 @@ public: \
     type##Id(): QUuid() {} \
     static type##Id create##type##Id() { return type##Id(QUuid::createUuid().toString()); } \
     static type##Id fromUuid(const QUuid &uuid) { return type##Id(uuid.toString()); } \
-};
+}; \
+Q_DECLARE_METATYPE(type##Id);
 
-DECLARE_TYPE_ID(Device)
-DECLARE_TYPE_ID(DeviceClass)
 
 DECLARE_TYPE_ID(Vendor)
+DECLARE_TYPE_ID(DeviceClass)
+DECLARE_TYPE_ID(Device)
 
 DECLARE_TYPE_ID(EventType)
 DECLARE_TYPE_ID(StateType)
diff --git a/server/jsonrpc/devicehandler.cpp b/server/jsonrpc/devicehandler.cpp
index c54339a0..86176264 100644
--- a/server/jsonrpc/devicehandler.cpp
+++ b/server/jsonrpc/devicehandler.cpp
@@ -40,7 +40,8 @@ DeviceHandler::DeviceHandler(QObject *parent) :
     setReturns("GetSupportedVendors", returns);
 
     params.clear(); returns.clear();
-    setDescription("GetSupportedDevices", "Returns a list of supported Device classes.");
+    setDescription("GetSupportedDevices", "Returns a list of supported Device classes, optionally filtered by vendorId.");
+    params.insert("vendorId", "o:uuid");
     setParams("GetSupportedDevices", params);
     QVariantList deviceClasses;
     deviceClasses.append(JsonTypes::deviceClassRef());
@@ -73,7 +74,7 @@ DeviceHandler::DeviceHandler(QObject *parent) :
     setParams("AddConfiguredDevice", params);
     returns.insert("success", "bool");
     returns.insert("errorMessage", "string");
-    returns.insert("deviceId", "uuid");
+    returns.insert("deviceId", "o:uuid");
     setReturns("AddConfiguredDevice", returns);
 
     params.clear(); returns.clear();
@@ -126,7 +127,7 @@ DeviceHandler::DeviceHandler(QObject *parent) :
     setParams("GetStateValue", params);
     returns.insert("success", "bool");
     returns.insert("errorMessage", "string");
-    returns.insert("value", "variant");
+    returns.insert("value", "o:variant");
     setReturns("GetStateValue", returns);
 
     // Notifications
@@ -158,10 +159,15 @@ QVariantMap DeviceHandler::GetSupportedVendors(const QVariantMap &params) const
 
 QVariantMap DeviceHandler::GetSupportedDevices(const QVariantMap &params) const
 {
-    Q_UNUSED(params)
     QVariantMap returns;
     QVariantList supportedDeviceList;
-    foreach (const DeviceClass &deviceClass, GuhCore::instance()->deviceManager()->supportedDevices()) {
+    QList<DeviceClass> supportedDevices;
+    if (params.contains("vendorId")) {
+        supportedDevices = GuhCore::instance()->deviceManager()->supportedDevices(VendorId(params.value("vendorId").toString()));
+    } else {
+        supportedDevices = GuhCore::instance()->deviceManager()->supportedDevices();
+    }
+    foreach (const DeviceClass &deviceClass, supportedDevices) {
         supportedDeviceList.append(JsonTypes::packDeviceClass(deviceClass));
     }
     returns.insert("deviceClasses", supportedDeviceList);
@@ -202,6 +208,7 @@ QVariantMap DeviceHandler::AddConfiguredDevice(const QVariantMap &params)
     switch(status) {
     case DeviceManager::DeviceErrorNoError:
         returns.insert("success", true);
+        returns.insert("errorMessage", "");
         returns.insert("deviceId", newDeviceId);
         break;
     case DeviceManager::DeviceErrorDeviceClassNotFound:
@@ -310,6 +317,7 @@ QVariantMap DeviceHandler::GetStateValue(const QVariantMap &params) const
     QVariant stateValue = device->stateValue(params.value("stateTypeId").toUuid());
 
     returns.insert("success", true);
+    returns.insert("errorMessage", "");
     returns.insert("value", stateValue);
     return returns;
 }
diff --git a/server/jsonrpc/jsonrpcserver.cpp b/server/jsonrpc/jsonrpcserver.cpp
index c0b606e8..5a443253 100644
--- a/server/jsonrpc/jsonrpcserver.cpp
+++ b/server/jsonrpc/jsonrpcserver.cpp
@@ -192,7 +192,7 @@ void JsonRPCServer::processData(const QUuid &clientId, const QByteArray &jsonDat
 
     QVariantMap returns;
     QMetaObject::invokeMethod(handler, method.toLatin1().data(), Q_RETURN_ARG(QVariantMap, returns), Q_ARG(QVariantMap, params));
-//    Q_ASSERT((targetNamespace == "JSONRPC" && method == "Introspect") || handler->validateReturns(method, returns));
+    Q_ASSERT((targetNamespace == "JSONRPC" && method == "Introspect") || handler->validateReturns(method, returns).first);
     sendResponse(clientId, commandId, returns);
 }
 
diff --git a/server/jsonrpc/jsontypes.cpp b/server/jsonrpc/jsontypes.cpp
index fa54ff6d..69403c7a 100644
--- a/server/jsonrpc/jsontypes.cpp
+++ b/server/jsonrpc/jsontypes.cpp
@@ -263,7 +263,10 @@ QPair<bool, QString> JsonTypes::validateMap(const QVariantMap &templateMap, cons
 {
     s_lastError.clear();
     foreach (const QString &key, templateMap.keys()) {
-        if (key != "params" && !map.contains(key)) {
+        if (templateMap.value(key).toString().startsWith("o:")) {
+            continue;
+        }
+        if (!map.contains(key)) {
             qDebug() << "missing key" << key << templateMap << map;
             QJsonDocument jsonDoc = QJsonDocument::fromVariant(map);
             return report(false, QString("Missing key \"%1\" in %2").arg(key).arg(QString(jsonDoc.toJson())));
@@ -279,20 +282,23 @@ QPair<bool, QString> JsonTypes::validateMap(const QVariantMap &templateMap, cons
 
 QPair<bool, QString> JsonTypes::validateProperty(const QVariant &templateValue, const QVariant &value)
 {
-    if (templateValue == "uuid") {
+    QString strippedTemplateValue = templateValue.toString();
+    strippedTemplateValue.remove(QRegExp("^o:"));
+
+    if (strippedTemplateValue == "uuid") {
         QString errorString = QString("Param %1 is not a uuid.").arg(value.toString());
         return report(value.canConvert(QVariant::Uuid), errorString);
     }
-    if (templateValue == "string") {
+    if (strippedTemplateValue == "string") {
         QString errorString = QString("Param %1 is not a string.").arg(value.toString());
         return report(value.canConvert(QVariant::String), errorString);
     }
-    if (templateValue == "bool") {
+    if (strippedTemplateValue == "bool") {
         QString errorString = QString("Param %1 is not a bool.").arg(value.toString());
         return report(value.canConvert(QVariant::Bool), errorString);
     }
-    qWarning() << QString("Unhandled property type: %1 (expected: %2)").arg(value.toString()).arg(templateValue.toString());
-    QString errorString = QString("Unhandled property type: %1 (expected: %2)").arg(value.toString()).arg(templateValue.toString());
+    qWarning() << QString("Unhandled property type: %1 (expected: %2)").arg(value.toString()).arg(strippedTemplateValue);
+    QString errorString = QString("Unhandled property type: %1 (expected: %2)").arg(value.toString()).arg(strippedTemplateValue);
     return report(false, errorString);
 }
 
@@ -339,6 +345,11 @@ QPair<bool, QString> JsonTypes::validateVariant(const QVariant &templateVariant,
                     qDebug() << "device not valid";
                     return result;
                 }
+            } else if (refName == vendorRef()) {
+                QPair<bool, QString> result = validateMap(vendorDescription(), variant.toMap());
+                if (!result.first) {
+                    qDebug() << "value not allowed in" << vendorRef();
+                }
             } else if (refName == deviceClassRef()) {
                 QPair<bool, QString> result = validateMap(deviceClassDescription(), variant.toMap());
                 if (!result.first) {
diff --git a/tests/auto/testjsonrpc.cpp b/tests/auto/testjsonrpc.cpp
index 70c4f396..98b41af4 100644
--- a/tests/auto/testjsonrpc.cpp
+++ b/tests/auto/testjsonrpc.cpp
@@ -46,6 +46,8 @@ private slots:
     void introspect();
 
     void getSupportedVendors();
+
+    void getSupportedDevices_data();
     void getSupportedDevices();
 
     void enableDisableNotifications_data();
@@ -217,14 +219,33 @@ void TestJSONRPC::getSupportedVendors()
     QCOMPARE(vendorId, guhVendorId);
 }
 
+void TestJSONRPC::getSupportedDevices_data()
+{
+    QTest::addColumn<VendorId>("vendorId");
+    QTest::addColumn<int>("resultCount");
+
+    QTest::newRow("vendor guh") << guhVendorId << 1;
+    QTest::newRow("no filter") << VendorId() << 1;
+    QTest::newRow("invalid vendor") << VendorId("93e7d361-8025-4354-b17e-b68406c800bc") << 0;
+}
+
 void TestJSONRPC::getSupportedDevices()
 {
-    QVariant supportedDevices = injectAndWait("Devices.GetSupportedDevices");
+    QFETCH(VendorId, vendorId);
+    QFETCH(int, resultCount);
+
+    QVariantMap params;
+    if (!vendorId.isNull()) {
+        params.insert("vendorId", vendorId);
+    }
+    QVariant supportedDevices = injectAndWait("Devices.GetSupportedDevices", params);
 
     // Make sure there is exactly 1 supported device class with the name Mock Wifi Device
-    QCOMPARE(supportedDevices.toMap().value("params").toMap().value("deviceClasses").toList().count(), 1);
-    QString deviceName = supportedDevices.toMap().value("params").toMap().value("deviceClasses").toList().first().toMap().value("name").toString();
-    QCOMPARE(deviceName, QString("Mock Device"));
+    QCOMPARE(supportedDevices.toMap().value("params").toMap().value("deviceClasses").toList().count(), resultCount);
+    if (resultCount > 0) {
+        QString deviceName = supportedDevices.toMap().value("params").toMap().value("deviceClasses").toList().first().toMap().value("name").toString();
+        QCOMPARE(deviceName, QString("Mock Device"));
+    }
 }
 
 void TestJSONRPC::enableDisableNotifications_data()